Fixes multiple embryos in single host

Added checks so you can’t have multiple embryos in a single host.
Checks are included in both facehugger.dm and alien_embryo.dm, because
there might be ways to implant embryos without facehuggers in the
future, but we also want to avoid creating and instantly deleting
objects.
Also changes chest augmentation surgery to remove any embryos. Before
you could remove the whole chest without finding the embryo inside.